An investigation of the effects of sub-lethal doses of inhibitors of methionine biosynthesis upon the growth and morphology of Lemna (duckweed) has demonstrated that these plants adapt to conditions of limiting methionine. Adaptation is evidenced by a transient period between 24 and 48 hours after initial exposure when individual colonies break apart abnormally and no new fronds appear, followed by resumption of frond emergence, restoration of a normal frond/colony ratio, and increased tolerance to the same or higher concentrations of the inhibitor. Investigation of wet weight and protein sulfur accumulation during exposure to 40 nM propargylglycine (PAG) showed that neither of these parameters undergoes a parallel transient effect, but instead slows to the "adapted" steady-state rate by 24 hours after first exposure. Studies of cystathionine Gamma-synthase activity during and after adaptation to PAG and/or lysine plus threonine suggest that adaptation is related, in part, to increases in the steady-state concentrations of this and other enzymes in the methionine pathway. The relationship between these changes and the morphological effects remains unclear.